Butterfly eating to the right, Tips?

I have been using manual mode, shutter priority, 1/500th. It seems you cannot change the iso with this mode from the default of 80? I cannot get enough dof and I don't know if 1/500 is really fast enough. I can only stop open to about f4.Any other tips on butterfly photography are welcome. I've got plenty of subject matter, about 4 of these and 5 or 6 big yellow and black ones at once on my pink and purple buddleias and white zinnias.